JP2010-144670A discloses an abnormality detecting device for an in-cylinder pressure sensor which detects the pressure inside a combustion chamber of an engine. The in-cylinder pressure sensor of this device includes a deforming part which deforms by the in-cylinder pressure, and a strain gage adhered to the deforming part. When the material composition of the deforming part is changed due to the influences of heat, etc. and the modulus of elasticity increases, the deforming part becomes difficult to be elastically deformed, and the gain of the output value of the in-cylinder pressure sensor decreases. Therefore, this device detects the gain of the output value of the in-cylinder pressure sensor, and when the gain is low, it concludes that the in-cylinder pressure sensor is abnormal.
In order to prevent a false diagnosis, when the gain of the output value of the in-cylinder pressure sensor is low and the output waveform is deviated in time, this device concludes that the engine is abnormal, and on the other hand, when the gain of the output value of the in-cylinder pressure sensor is low and the output waveform is not deviated in time, it concludes that the in-cylinder pressure sensor is not abnormal.
The present inventors have determined the gain of the output value of the in-cylinder pressure sensor, as a result of diligent examinations. Specifically, although the detailed mechanism is unknown, it was newly determined that the symmetry of the signal value of the in-cylinder pressure sensor collapses, when the in-cylinder pressure sensor has failed by being damaged under the influence of heat etc. In this case, a difference between the signal value of the in-cylinder pressure sensor at the timing where the crank angle is advanced by a given amount from a compression top dead center, and the signal value of the in-cylinder pressure sensor at the timing where the crank angle is retarded by the same amount from the compression top dead center increases.
Although the technology disclosed in JP2010-144670A takes the gain of the output value of the in-cylinder pressure sensor into consideration, since it does not take the symmetry of the output value at all into consideration, the present inventors have noticed that there is room for improvement for the accuracy of the abnormality diagnosis.